1. Field of the Invention
The present invention relates to a method for manufacturing a double-sided flexible printed board using a polyimide insulating layer.
2. Related Art of the Invention
The following boards are commonly used as conventional polyimide-based double-sided flexible printed boards: (i) boards comprising copper foil, a thermoplastic polyimide adhesive, a non-thermoplastic polyimide film, and a thermoplastic polyimide adhesive and copper foil; (ii) boards comprising copper foil, a thermoplastic polyimide adhesive layer, and another copper foil, where the (i) and (ii) boards are fabricated by high-temperature pressing, heat lamination, or the like into a layered structure and (iii) boards fashioned by techniques in which a non-thermoplastic polyimide is cast-molded on copper foil and then a copper layer is formed by sputtering followed by electrolytic plating on the non-thermoplastic polyimide yield a layered structure that comprises copper foil, a non-thermoplastic polyimide film, and a copper layer comprised of a copper sputtered thin film and an electrolytic copper plating film formed thereon.
The double-sided flexible printed boards in (i) are disadvantageous, however, in that they have inadequate heat resistance because of the thermoplastic nature of the adhesive. The double-sided flexible printed boards in (ii) are disadvantageous in that their heat resistance is inadequate because of the thermoplastic nature of the adhesive layer and that their dimensional stability is inadequate because shrinkage is apt to occur during heating. The double-sided flexible printed boards in (iii) are disadvantageous in that the electrolytic copper plating layer containing a copper sputtered thin film on the non-thermoplastic polyimide film has relatively low peel strength, unsatisfactory reliability, and inferior long-term aging characteristics under conditions of heating and humidification.
To overcome these shortcomings, it has recently been proposed to use double-sided flexible printed circuit boards (metal foil/polyimide/metal foil) fabricated by a method in which a highly wettable polyamic acid varnish is applied to each of two metal foil and dried to give two laminates (each having such a polyamic acid layer); each metal foil is patterned, and the polyamic acid layers are imidated while laminated under conditions of high temperature and pressure to form a polyimide insulating layer (Japanese Patent No. 2746643). Such double-sided flexible printed circuit boards have excellent heat resistance and dimensional stability, develop adequate adhesion between the polyimide layer and the metal foil on both sides, and possess proper long-term aging characteristics.
When, however, the polyamic acid layers are laminated each other under conditions of high temperature and pressure following metal foil patterning, there is a danger that the moisture generated during imidation will corrode the metal foil without evaporating from the polyimide layer. Another drawback is that such double-side patterning impairs handling and results in reduced manufacturing efficiency. Yet another drawback is the need to provide high positional accuracy for lamination.